Conventionally, an antenna for emitting and receiving an electromagnetic wave may be produced by associating a waveguide with a radiating element that may, for example, take the form of a horn. A horn-shaped waveguide, more simply called a horn, has a rectangular transverse (i.e. perpendicular to the propagation direction of the wave) cross section that gradually increases towards the aperture. Such a waveguide makes it possible to promote propagation, along its longitudinal axis, of an electromagnetic wave polarised along an axis orthogonal to the longitudinal axis of the horn. The electric field of the electromagnetic wave may be decomposed into a component parallel to the shortest sides of the aperture, and into a component parallel to the longest sides of the aperture. The first component is called the principal or co-polarisation component. The other component is called the cross-polarisation component. In the context of certain applications, it is desirable to reduce as much as possible the amplitude of the cross-polarisation component. One solution consists in placing a grating over the aperture of the horn. A grating is generally made of a metal, for example aluminium. It is formed from a set of plates placed parallel to the longest sides of the aperture of the waveguide. The grating allows the co-polarisation component to pass through and filters the cross-polarisation component of an electromagnetic wave. For a relatively directional horn, for example with a gain higher than 25 dBi, equipped with a grating, it is possible to obtain a cross-polarisation component the amplitude of which is about 40 to 45 dB smaller than the amplitude of the co-polarisation component. However, the effectiveness of the filtering very clearly or even completely decreases when the horn is less directional. This is notably the case for the test horns used in microwave anechoic chambers. Also, the filtering is effective only over a narrow frequency band. With the increasing demand for better antenna performance, it is becoming useful to develop horns providing an attenuation of the cross-polarisation component of at least 40 dB relative to the co-polarisation component, and this over widened frequency bands, for example by about 40% to 50%.